The .DELTA.9 desaturase of cyanobacterium is an enzyme converting stearic acid linking to glycerolipid to oleic acid, and converting palmitic acid linked to C-1 of glycerol to palmitoleic acid.
In the cyanobacterium, the desaturation process of fatty acids has been shown to be initiated through the induction of the double bond into a carbon chain at .DELTA.9 position, followed by .DELTA.12 and then .DELTA.6 or .DELTA.15. The .DELTA.9 desaturase is an important enzyme which is responsible for the first step of a series of desaturation reactions, and is associated with the reaction of introducing the double bond into a carbon chain of stearic acid or palmitic acid at .DELTA.9, which are linked to glycerolipid. This reaction requires reducing power, which depends on ferredoxin and NADPH.
On the other hand, an enzyme introducing the double bond into stearic acid at .DELTA.9, which is not linked to glycerolipid, has been reported as stearoyl CoA desaturase in cytoplasm of animals and as stearoyl ACP (acyl-carrier protein) desaturase in chloroplast of plants. The DNA sequence of these enzymes has been determined.
The .DELTA.9 desaturase of cyanobacteria is characterized by converting palmitic acid or stearic acid linking to glycerolipid to an unsaturated fatty acid, while the above two .DELTA.9 desaturases can not catalyze this reaction. To appreciate the determinating factors of its substrate specificity, .DELTA.9 desaturase of several species of cyanobacterium should be analyzed at the molecular level.
The phase transition temperature of biomembranes is dependant on the content of unsaturated fatty acids in the polar lipid membrane; therefore, the phase transition temperature falls as the content of an unsaturated fatty acids increases. It has been reported that the amount of unsaturated fatty acids in cyanobacterium increases due to the lower temperature, suggesting that the composition of fatty acids in cell membrane is also associated with the low-temperature tolerance of plants. Thus, the expression of fatty acid desaturase is considered to be adjusted by low temperature. Approaches to the elucidation of the mechanisms of adjustment of expression demand isolating the associated gene(s).
For these reasons, the isolation of the gene of .DELTA.9 desaturase of cyanobacteria has been required, however, there has been no report of the isolation of this gene with an exception of the isolation from Anabaena variabilis.